Conventionally, a thyristor is made to conduct when the voltage across its terminals becomes greater than a trigger voltage.
Further to the trigger voltage, another important parameter of a thyristor is the holding voltage, that is to say the minimum voltage such that the thyristor remains conductive after having been triggered.
In certain applications, thyristors have a high trigger voltage, for example around 3.6 V, but a low holding voltage, for example around 1.2 V, which may then be less than the rated supply voltage of the integrated circuit incorporating such thyristors. This is the case, for example, with an integrated circuit having a supply voltage of 3.3 V.
Consequently, during operation of the integrated circuit, the thyristors may be triggered and become conductive during an electrical overstress (“Electrical OverStress”: EOS) and then continue to conduct until they are destroyed, because the supply voltage of the circuit is then always greater than the holding voltage of these thyristors.
One solution, based on a protective device having a structure of three cascoded thyristors, is generally proposed in order to increase the holding voltage of such a protective device.
However, such a structure with three thyristors also increases the trigger voltage and the surface occupancy of the device on silicon.